PNP biasingFor a PNP transistor, the situation is just a “mirror image” of the case for an NPN device.The diodes are turned around the opposite way, the arrow points inward rather thanoutward in the transistor symbol, and all the polarities are reversed. The dual-diodePNP model, along with the actual bipolar transistor circuit, are shown in Fig. 22-5. Inthe discussion above, simply replace every occurrence of the word “positive” with theword “negative.”You need not be concerned with what actually goes on inside the semiconductormaterials in NPN and PNP transistors. The important thing is the fact that either typeof device can serve as a sort of “current valve. “ Small changes in the base voltage, EB,cause small changes in the base current, IB. This induces large fluctuations in the cur-rent IC through the transistor.In the following discussion, and in most circuits that appear later in this book, you’llsee NPN transistors used almost exclusively. This doesn’t mean that NPN is better thanPNP; in almost every case, you can replace each NPN transistor with a PNP, reverse thepolarity, and get the same results. The motivation is to save space and avoid redundancy.Biasing for current amplificationBecause a small change in the base current, IB, results in a large collector-current (IC)variation when the bias is just right, a transistor can operate as a current amplifier. Itmight be more technically accurate to say that it is a “current-fluctuation amplifier,” be-cause it’s the magnification of current variations, not the absolute current, that’s important.404 The bipolar transistor22-4Relative collectorcurrent (IC) as afunction of base voltage(EB) for a hypotheticalsilicon transistor.